The present invention relates generally to thin-film electroluminescent devices and more particularly relates to thin-film electroluminescent device where bandgap engineering has been used to design the device.
The operational efficiency of today""s alternating-current thin-film electroluminescent (ACTFEL) display depends on three main factors, which arc the radiative efficiency, the outcoupling efficiency of the emitted photons, and the excitation efficiency of dopant centers. (See R. Mach and G. O. Muller, Phys. Stat. Sol. (a) 81, 609 (1984).) These three components of the total efficiency must be increased to meet the brightness requirements for full-color applications and to improve the operational parameters of a monochrome (typically, ZnS:Mn-based) display. The radiative efficiency is determined mainly by the recombination cross-section of the luminescent centers. A straightforward increase of the dopant concentration results in higher concentration quenching of luminescence. (See P. De Visschere and K. Neyts, J. Luminescence 52, 313 (1992)) The improvement of the photon outcoupling efficiency is mainly associated with the optical matching of the individual layers of the device. Difficulties in obtaining a significant increase in the excitation efficiency of luminescent centers are generally attributed to the low efficiency of the tunneled electrons, caused partially by space charge formation and a lack of electron acceleration due to short electron mean-free path (the distances required for electrons to gain enough energy in ZnS to impact excite Mn centers and to ionize the lattice are 16 and 40 nm, respectively). (See for example, A. N. Krasnov, P. G. Hofstra, and M. T. McCullough, J. Vac. Sci. Tech. (a) 18, 671 (2000).) This leads to a decreasing impact ionization probability with the distance from the insulator-semiconductor interface (ISI) to the middle of the phosphor layer. As a result, only the phosphor region adjacent to the cathodic ISI is efficiently involved in light emission. (See J. Benoit, C. Barthou and P. Benalloul, J. Appl. Phys. 73, 1435 (1993).) All attempts to increase the impact ionization probability of the kinetic electrons through the phosphor bandgap Eg reduction have so far shown little success, mainly due to a decreased energy of the tunneled electrons or temperature quenching problems. (See R. Mach, J. von Kalben, G. O. Muller, W. Gericke, and G. U. Reinsperger, J. Appl. Phys. 54, 4657 (1983).)
According to an aspect of the present invention there is provided an alternating current thin-film electroluminescent display having two stacked dielectrics a semiconductor active layer therebetween, and metallic cladding electrodes at each side thereof; wherein the semiconductor layer is developed by automated thermal co-evaporation so as to provide a monotonic decrease of the band gap thereof from the respective interfaces with said stacked dielectrics to the middle of said semiconductor active layer so that the dopant concentration thereof is maintained at about 0.7%.